Active solid-state semiconductor devices, and in particular, silicon photodiodes, are among the most popular photodetectors having a sufficiently high performance over a large wavelength range and a sufficient ease of use. Silicon photodiodes are sensitive to light in the wide spectral range, extending from deep ultraviolet through visible to near infrared, which is approximately 200 nm to 1100 nm. Silicon photodiodes, by using their ability to detect the presence or absence of minute light intensities, facilitate the precise measurement of these minute light intensities upon appropriate calibration. For example, appropriately calibrated silicon photodiodes detect and measure light intensities varying over a wide range, from very minute light intensities of below 10−13 watts/cm2 to high intensities above 10−3 watts/cm2.
Silicon photodiodes can be employed in an assortment of applications including, but not limited to, spectroscopy, distance and speed measurement, laser ranging, laser guided missiles, laser alignment and control systems, optical free air communication, optical radar, radiation detection, optical position encoding, film processing, flame monitoring, scintillator read out, environmental applications such as spectral monitoring of earth ozone layer and pollution monitoring, low light-level imaging, such as night photography, nuclear medical imaging, photon medical imaging, and multi-slice computer tomography (CT) imaging, security screening and threat detection, thin photochip applications, and a wide range of computing applications.
Typically, photodiode arrays employ a scintillator material for absorbing high energy (ionizing) electromagnetic or charged particle radiation, which, in response, fluoresces photons at a characteristic wavelength. Scintillators are defined by their light output (number of emitted photons per unit absorbed energy) short fluorescence decay times, and optical transparency at wavelengths of their own specific emission energy. The lower the decay time of a scintillator, that is, the shorter the duration of its flashes of fluorescence are, the less so-called “dead time” the detector will have and the more ionizing events per unit of time it will be able to detect. Scintillators are used to detect electromagnetic waves or particles in many security and detection systems, including CT, X-ray, and gamma ray. There, a scintillator converts the energy to light of a wavelength which can be detected by photomultiplier tubes (PMTS) or P-N junction photodiodes.
Photodiodes are typically characterized by certain parameters, such as, among others, electrical characteristics, optical characteristics, current characteristics, voltage characteristics, and noise. Electrical characteristics predominantly comprise shunt resistance, series resistance, junction capacitance, rise or fall time and/or frequency response. Optical characteristics comprise responsivity, quantum efficiency, non-uniformity, and/or non-linearity. Photodiode noise may comprise, among others, thermal noise, quantum, photon or shot noise, and/or flicker noise.
In an effort to increase the signal to noise ratio and enhance the contrast of the signal, it is desirable to increase the light-induced current of photodiodes. Thus, photodiode sensitivity is enhanced while the overall quality of the photodiode is improved. Photodiode sensitivity is crucial in low-level light applications and is typically quantified by a parameter referred to as noise equivalent power (NEP), which is defined as the optical power that produces a signal-to-noise ratio of one at the detector output. NEP is usually specified at a given wavelength over a frequency bandwidth.
Photodiodes absorb photons or charged particles, facilitating detection of incident light or optical power and generating current proportional to the incident light, thus converting the incident light to electrical power. Light-induced current of the photodiode corresponds to the signal while “dark” or “leakage” current represents noise. “Dark” current is that current that is not induced by light, or that is present in the absence of light. Photodiodes process signals by using the magnitude of the signal-to-noise ratio.
Leakage current is a major source of signal offset and noise in current photodiode array applications. Leakage current flows through the photodiode when it is in a “dark” state, or in the absence of light at a given reverse bias voltage applied across the junction. Leakage current is specified at a particular value of reverse applied voltage. Leakage current is temperature dependent; thus, an increase in temperature and reverse bias results in an increase in leakage or dark current. A general rule is that the dark current will approximately double for every 10° C. increase in ambient temperature. It should be noted, however, that specific diode types can vary considerably from this relationship. For example, it is possible that leakage or dark current will approximately double for every 6° C. increase in temperature.
In certain applications, it is desirable to produce optical detectors having small lateral dimensions and spaced closely together. For example in certain medical applications, it is desirable to increase the optical resolution of a detector array in order to permit for improved image scans, such as computed tomography (CT) scans. However, at conventional doping levels utilized for diode arrays of this type, the diffusion length of minority carriers generated by photon interaction in the semiconductor is in the range of at least many tens of microns, and such minority carriers have the potential to affect signals at diodes away from the region at which the minority carriers were generated.
Thus, an additional disadvantage with conventional photodiode arrays is the amount and extent of crosstalk that occurs between adjacent detector structures, primarily as a result of minority carrier leakage current between diodes. The problem of crosstalk between diodes becomes even more acute as the size of the detector arrays, the size of individual detectors, the spatial resolution, and spacing of the diodes is reduced.
Various approaches have been used to minimize such crosstalk including, but not limited to, providing inactive photodiodes to balance the leakage current, as described in U.S. Pat. Nos. 4,904,861 and 4,998,013 to Epstein et al., the utilization of suction diodes for the removal of the slow diffusion currents to reduce the settling time of detectors to acceptable levels, as described in U.S. Pat. No. 5,408,122, and providing a gradient in doping density in the epitaxial layer, as described in U.S. Pat. No. 5,430,321 to Effelsberg.
In addition to leakage current and effects of crosstalk, noise is often a limiting factor for the performance of any device or system. In almost every area of measurement, the limit to the detectability of signals is set by noise, or unwanted signals that obscure the desired signal. As described above, the NEP is used to quantify detector noise. Noise issues generally have an important effect on device or system cost. Conventional photodiodes are particularly sensitive to noise issues. Like other types of light sensors, the lower limits of light detection for photodiodes are determined by the noise characteristics of the device.
As described above, the typical noise components in photodiodes include thermal noise; quantum or shot noise; and flicker noise. These noise components collectively contribute to the total noise in the photodiode. Thermal noise, or Johnson noise, is inversely related to the value of the shunt resistance of photodiode and tends to be the dominant noise component when the diode is operated under zero applied reverse bias conditions. Shot noise is dependent upon the leakage or dark current of photodiode and is generated by random fluctuations of current flowing through the device, which may be either dark current or photocurrent. Shot noise tends to dominate when the photodiode is used in photoconductive mode where an external reverse bias is applied across the device. As an example, detector noise generated by a planar diffused photodiode operating in the reverse bias mode is a combination of both shot noise and thermal noise. Flicker noise, unlike thermal or shot noise, bears an inverse relationship to spectral density. Flicker noise may dominate when the bandwidth of interest contains frequencies less than 1 kHz.
Secondary issues also contribute to dark noise and other noise sources that impact photodiode sensitivity. These include primarily determination and/or selection of apt active area specifications (geometry and dimensions), response speed, quantum efficiency at the wavelength of interest, response linearity, and spatial uniformity of response, among others.
Further, when a photodiode is used in conjunction with an electronic amplifier, the capacitance of the photodiode can be a dominating contributing characteristic in the overall noise. It is therefore desirable to design and fabricate photodiodes that have low overall capacitance.
In CT applications, such as those employed for baggage screening, it is desirable to have high density photodiode arrays with low dark current, low capacitance, high signal-to-noise ratio, high speed and low crosstalk.
As mentioned above, however, there are numerous problems with conventional photodiodes that attempt to achieve these competing and often conflicting characteristics. Referring now to FIGS. 1a and 1b, top and cross-sectional views, respectively, of a conventional photodiode are shown. The photodiode shown in FIGS. 1a and 1b is typical in that it is fabricated such that the p+ diffused area has the same dimensions as the scintillator crystal. For example, and as described in greater detail below, if the scintillator crystal has a length of 2 mm and a width of 2 mm, then the p+ diffused area is also 2 mm×2 mm. This is a typical design characteristic for photodiodes used in x-ray/scintillator applications.
Referring to FIG. 1a, conventional photodiode 100 comprises substrate material 102, which is typically a bulk silicon substrate lightly doped with a suitable impurity of a selected conductivity type, such as p-type or n-type. In order to meet desired capacitance performance specification, the device is fabricated on a n-type silicon substrate wafer that is of high resistivity, typically on the order of 4500-9000 ohm cm. The photodiode comprises a shallow p+ diffused region 110, deep p+ diffused region 112, and anode metal pads 114a for forming at least one contact and 114b for forming at least one wire bond. It should be noted that, in a typical photodiode chip, distance 129 between the p+ diffused area 110 and the edge 130 of the photodiode 100 ranges from 0.1 mm to 0.125 mm.
Referring to FIG. 1b, conventional photodiode 100 comprises a shallow p+ diffused region 110, which has a top surface area of 2 mm×2 mm. Conventional photodiode 100 further comprises a scintillator crystal 120, which is of the same dimensions as the p+ active area/diffused region 110 as described above, or 2 mm×2 mm, mounted on the photodiode using an epoxy. Conventional photodiode 100 also comprises deep p+ diffused region 112; anode metal pads 114a for forming at least one contact and 114b for creating at least one wire bond; and a layer 115 comprised of silicon oxide/silicon nitride that acts as junction passivation and antireflection layer.
The conventional photodiode shown in FIGS. 1a and 1b are, however, fabricated on an expensive, high resistivity silicon substrate material 102 to accommodate for a large p+ diffused area while still retaining low overall capacitance and thus, improved performance. Further, a large p+ diffused area has high dark current since the amount of dark current is proportional to the size of the p+ diffused area, and thus, a low signal to noise ratio. Still further, because the p+ diffused area is large, there is little distance between the p+ diffused area and the edge of the chip, there is an increase in sawing damage and thus, overall yield may be relatively low.
Thus, conventional photodiodes suffer from low signal-to-noise ratios and sub-optimal characteristics. Conventional photodiodes are also known to have high cost of manufacturing and low yield.
Therefore, what is needed is a photodiode and photodiode array that can be manufactured on a low resistivity substrate wafer.
What is also needed is a photodiode and photodiode array that can be manufactured with a relatively small p+ diffused active area to obtain low capacitance while still retaining high signal current.
In addition, there is need in the art for photodiodes that have improved overall performance characteristics, allow for high yield and can be inexpensively manufactured.